> For the complete documentation index, see [llms.txt](https://www.oulan.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.oulan.com/2017/02/21.md).

# 21

![](https://385430918-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M8iFCzaIpXf8zXqpE6I%2F-MTY4T8qgtjCI5ScVmTS%2F-MTY4zAX3d9iM4Q1JvDc%2Fimage.png?alt=media\&token=dd3540b2-8ac9-4024-9425-afcc2bcea3cf)